To restrict the client accessing the Webservice. (He was not satisfied :( The answer is: Concurrency is of three types: 1. Single - Only once client can assess the service. Uses SyncLock. 2. Multiple: Multiple Client can access the web service. It does not use SyncLock. 3. Reentrant - A single request thread has access to the WCF service object, but the thread can exit the WCF service to call another WCF service or can also call WCF client through callback and reenter without deadlock.)